Phosphodiesterase (PDE) is known as an important enzyme which inactivates cyclic nucleotides (cAMP and cGMP) being second messengers by hydrolyzing one side of phosphodiester bonds present in the cyclic nucleotides. PDEs which break down cAMP are classified into several isoforms. Among these, type IV phosphodiesterase (PDE4) is one of the principal cAMP-breaking enzymes present in many inflammatory cells, immune cells, blood, and organs such as the brain and the lungs (Non-Patent Document 1: International Journal of Chronic Obstructive Pulmonary Disease, 2008, 3(4), pp. 543-561).
A PDE4 inhibitor reduces the production and the release of various inflammatory cytokines such as TNF-α and IL-23, and therefore is known to be effective for the treatment of various inflammatory diseases involving TNF-α and the like (Non-Patent Document 2: Expert Opinion on Investigational Drugs, 2002, 11(1), pp. 1-13). In addition, the PDE4 inhibitor is reported to have an effect not only on respiratory inflammatory diseases (asthma and chronic obstructive pulmonary disease (COPD)) but also on dermal inflammatory diseases (psoriasis and atopic dermatitis), and moreover is reported to have a potential to produce an effect on mental illnesses such as depressive disorder and dysmnesia (see Non-Patent Document 2). Furthermore, in animal models, the PDE4 inhibitor is also suggested to be effective on interstitial pneumonia such as idiopathic pulmonary fibrosis (Non-Patent Document 3: British Journal of Pharmacology, 2009, 156, pp. 534-544). What is more, the PDE4 inhibitor is suggested to be effective on various fibrosis (Non-Patent Document 4: Journal of Cellular Physiology, 2011, 226, pp. 1970-1980) and on systemic sclerosis being a type of fibrosis (Non-Patent Document 5: Annals of the Rheumatic Diseases 2017, 76, pp. 1133-1141) because the PDE4 is involved in the functions of fibroblasts.
It has been known in recent years that expression levels of the PDE4 are excessively high in cells derived from patients with colorectal cancer, and its inhibitor has antitumor activity (Non-Patent Document 6: Molecular Cancer, 2012, 11:46). There has been also reported that the PDE4 is involved in the growth of lung cancer and angiogenesis, and its inhibitor has a potential to produce an effect on lung cancer as well (Non-Patent Document 7: Oncogene, 2013, 32, pp. 1121-1134) and is a possible candidate for a promising method for treating brain tumors (Non-Patent Document 8: Trends in Pharmacological Sciences, June 2011, Vol. 32, No. 6, pp. 337-344) and acute lymphocytic leukemia (Non-Patent Document 9: Blood, May 2002, Vol. 99, No. 9, pp. 3390-3397), for example. Thus, the PDE4 inhibitor might be effective on various types of cancer.
There has been also reported that the PDE4 inhibitor might be effective on metabolic diseases such as obesity and diabetes (Non-Patent Document 10: Cell 148, February 2012, pp. 421-433) and on cognitive disorders caused by aging, Alzheimer's disease, Parkinson's disease, schizophrenia, and Huntington's disease (Non-Patent Document 11: Expert Opinion on Therapeutic Targets, September 2013, 17(9), pp. 1011-1027).
Theophylline, which is known as a non-selective PDE inhibitor, has conventionally been used for the treatment of asthma (Non-Patent Document 12: British Journal of Pharmacology, 2008, 155, pp. 308-315). In addition, ibudilast, a non-selective PDE inhibitor, shows a treatment effect on bronchial asthma and cerebrovascular diseases thanks to its anti-inflammatory action and vasodilation action (Non-Patent Document 13: Expert Opinion on Pharmacotherapy, 2009, 10, pp. 2897-2904) and is used for allergic conjunctivitis (eye lotion) (Non-Patent Document 14: Eye Contact Lens, September 2009, Vol. 35, No. 5, pp. 251-254). Ibudilast is also expected to produce an effect on multiple sclerosis and neuropathic pain (Non-Patent Document 15: Expert Opinion on Investigational Drugs, 2016, Vol. 25, No. 10, pp. 1231-1237). Roflumilast, which is a strong oral PDE4 inhibitor, has been approved and used in Europe and the United States, as a drug applied to chronic obstructive pulmonary disease (COPD) (Non-Patent Document 16: British Journal of Pharmacology, 2011, 163, pp. 53-67). In animal models, roflumilast has been also suggested to be effective on interstitial pneumonia such as idiopathic pulmonary fibrosis (see Non-Patent Document 3). Moreover, clinical trials have been carried out for obesity, dementia, and atopic dermatitis (Non-Patent Document 17: Nature Reviews Drug Discovery, April 2014, Vol. 13, pp. 290-314), and alopecia areata (Non-Patent Document 18: Summer Meeting of the American Academy of Dermatology (AAD) 2016, Abstract 4070). Furthermore, an effect has been reported of lowering blood sugar levels of patients with diabetes (Non-Patent Document 19: The Journal of Clinical Endocrinology & Metabolism, September 2012, 97(9), pp. 1720-1725). In recent years, apremilast, which is an oral PDE4 inhibitor too, has been approved and widely used in Europe and the United States as a medication for the treatment of psoriatic arthritis and psoriasis vulgaris, and its clinical trials have been carried out for acne, ankylosing spondylitis, rheumatism, Behget's disease, and atopic dermatitis (Non-Patent Document 17 and Non-Patent Document 20: Drugs, 2014, 74, pp. 825-837). Clinical trials of crisaborole, which is a topical PDE4 inhibitor, have been reported in that crisaborole has an effect on atopic dermatitis of children and adults (Non-Patent Document 21: Journal of the American Academy of Dermatology, September 2016, Vol. 75, No. 3, pp. 494-503). Clinical trials of the PDE4 inhibitor have been carried out for various inflammatory diseases (inflammatory bowel disease, Crohn's disease, multiple sclerosis, rheumatism, sarcoidosis, Behget's disease, and rhinitis) in addition to the above. Also, non-clinical studies have also pointed out a possibility that the PDE4 inhibitor might be effective on systemic lupus erythematosus. Further, many compounds as the PDE4 inhibitors have a central action and have been reported to have an effect on depressive disorder, Parkinson's disease, learning disability, dysmnesia, and Alzheimer's disease (Non-Patent Document 2 and Non-Patent Document 22: BMC Medicine, 2013, 11:96).
As described above, diseases on which the PDE4 inhibitor might produce effects include asthma, COPD, interstitial pneumonia, various fibrosis such as idiopathic pulmonary fibrosis and systemic sclerosis, inflammatory bowel diseases such as Crohn's disease, multiple sclerosis, rheumatism, ankylosing spondylitis, acne, atopic dermatitis, alopecia areata, allergic conjunctivitis, rhinitis, psoriatic arthritis, psoriasis vulgaris, sarcoidosis, Behget's disease, systemic lupus erythematosus, cerebrovascular disease, neuropathic pain, depressive disorder, cognitive disorders, learning disability, Parkinson's disease, Alzheimer's disease, Huntington's disease, schizophrenia, various types of cancer (such as colorectal cancer, lung cancer, hematologic cancer, and brain tumor), and metabolic diseases (such as diabetes and obesity).
Japanese Patent Application Publication No. Hei 6-345744 (Patent Document 1) and Japanese Patent Application Publication No. Hei 10-29987 (Patent Document 2) have reported the activity of a benzoxazole derivative on serotonin 5-HT3. Additionally, International Publication No. Wo 2015/005429 (Patent Document 3) has reported a benzoxazole derivative which has a PDE4 inhibitory activity. However, the substituent at position 2 in each of these benzoxazole derivatives is a monocyclic heterocycle.